This invention relates to the field of water treatment for purification or for making it drinkable, and particularly but not exclusively urban or industrial waste water and rain water.
Standard practice includes various physicochemical water treatment techniques making use of different types of settling tanks that can achieve different hydraulic treatment velocities that depend particularly on their architecture and their size.
With simple settling tanks it is hardly possible to achieve velocities higher than 2.5 m/h, but these velocities can be as high as 10 to 15 m/h when the internal structure of the settling tanks used includes lamellar elements. In particular, lamellar settling tanks of this type are marketed by the Applicant under the trade name MULTIFLO (registered trademark). Furthermore, when the settling step itself is preceded by a coagulation-flocculation step, velocities of the order of 30 m/h, or even 80 m/h, can be achieved if a sludge recirculation step is included. An even more advanced technique developed by the Applicant and marketed under the brand ACTIFLO (registered trademark) consists of using settling tanks with flocs ballasted by appropriate ballasting material, and particularly micro-sand. A technique of this type is described in detail in patents FR 2 627 704 and FR 2 719 734 issued to the Applicant. With this type of ballasting of flocs formed in settling tanks, treatment velocities of 50 m/h and up to 200 m/h can be achieved.
Settling tanks of the type described above produce unthickened sludge. The sludge concentration at the outlet from ballasted floc settling tanks using a granular material usually varies between 0.5 g/l and 20 g/l of dry matter (DM). Therefore, this sludge has to be thickened using thickeners to achieve a concentration of between 20 and 250 g/l of DM (corresponding to a dryness ratio of 2 to 25%), and preferably between 20 and 90 g/l of DM (corresponding to a dryness ratio of 3 to 9%).
Within the framework of the ACTIFLO technique described above, a thickener is designed to recover sludge from the settling tank, and overflow water from thickeners is returned to the top of the settling tank. This return of water from the thickener overflow to the beginning of the line increases the water influent entering the installation to be treated.
In other settling tanks according to the standard practice, the sludge thickening function is built into the settling tanks themselves. These settling tanks are then conventionally called xe2x80x9cthickener settling tanksxe2x80x9d. In this respect, note the technique also developed by the applicant and marketed under the name MULTIFLO E or EF, or many other techniques described in patent document FR 2 156 277. Similar techniques also use sludge recirculation (U.S. Pat. No. 4, 388, 195, FR 2 553 082). With these thickener settling tanks, hydraulic treatment velocities of the order of 30 to 50 m/h, or even 80 m/h in some cases, can be obtained to produce a sludge with a dryness that can vary from 2 to 25%, with an average of 3 to 8% (corresponding to 30 to 80 g/l of dry matter). However, they cannot obtain velocities as high as the velocities possible with ballasted floc settling tanks.
Concerning the technique used for thickeners, static thickeners based on a load on the base mat corresponding to the maximum allowable mass flow of dry matter per unit area. Conventionally, this base mat load is between 20 and 100 kg/DM/m2/d. A lamellar thickener preceded by an unthickened sludge flocculation step can increase base mat loads to between 500 and 3000 kg DM/m2/d, and sometimes more.
The main purpose of this invention is to describe a process capable of benefiting from the advantages of ballasted floc settling tanks with a granular material that can operate at very high hydraulic velocities (up to 200 m/h as mentioned above) and the advantages of lamellar thickener settling tanks that give very good thickening of the sludge, but the maximum hydraulic velocities are not as high as the velocities possible with ballasted floc settling tanks.
Another purpose of this invention is to describe such a process making use of settling tanks with base mat areas smaller than the areas that would conventionally be used according to standard practice for optimized hydraulic treatment velocities and sludge drynesses, and therefore reduce the size of the corresponding installations for embodiment of this process.
Another objective of this invention is to divulge such a process that can overcome the need to return water overflowing from thickeners to the beginning of the installation.
Another objective of this invention is to propose a process that can enable efficient treatment of water when the flow or mass content of the water to be treated increases, even suddenly, for example during rainstorms.
These various objectives are achieved according to the invention which relates to a process for the physicochemical treatment of water, characterized in that it comprises at least one step that consists of passing the water to be treated in at least one settling tank of the type that produces unthickened sludge, and in at least one thickener settling tank, the said thickener settling tank treating the sludge from the two settling tanks and part of the water to be treated.
Therefore the main innovation of this process according to this invention is the characteristic according to which the thickener settling tank is not used only to treat sludge from a settling tank producing thickened sludge, but it is also used to treat some of the water itself (influent). As described in detail later, it is thus possible to reduce the size of the installations since the settling tank function of the thickener settling tank can increase the available settling area to treat the influent.
According to one particularly interesting variant of the invention, the said settling tank of the type producing unthickened sludge is a ballasted floc settling tank ballasted by at least one granular material, preferably micro-sand, and the said thickener settling tank is a lamellar thickener settling tank. The granular material will preferably consist of micro-sand, although almost any granular material known to those skilled in the art could be envisaged.
The invention thus combines the advantages of settling tanks with flocs ballasted by a granular material capable of functioning at very high hydraulic velocities (up to 200 m/h as mentioned above), and the advantages of lamellar thickener settling tanks that provide a very good sludge thickening function. Thus, in a preferred variant of the invention, ACTIFLO ballasted floc settling tanks can be used with MULTIFLO E or EF thickener settling tanks marketed by the Applicant and described above. Within the framework of this type of preferred embodiment, it will be noted that the sludge originating from such a settling tank with flocs ballasted by a granular material thickens very well and does not cause any disturbance in the thickener settling tank. All that is necessary to achieve this is to reduce the water flow treated on this structure, and increase the water flow treated on the settling tank with flocs ballasted by the granular material.
The size of a thickener settling tank depends on two parameters, namely the hydraulic velocity for settling and the mass load on the base mat for thickening. The hydraulic velocity is usually the controlling factor. With the process according to the invention, returning unthickened sludge from the lamellar settling tank with flocs ballasted by the granular material, the concentration of sludge in the thickener settling tank increases such that the maximum allowable hydraulic velocity can be increased, and thus the maximum hydraulic velocity corresponding to the velocity of the mass load on the base mat can be used. Therefore, with the process according to the invention, the thickener settling tank is sized as a function of the maximum allowable load on the base mat and the ballasted floc settling tank is sized to handle the additional amount that cannot be accepted hydraulically by the thickener settling tank.
According to another embodiment of the invention, the lamellar thickener settling tank used may also include sludge recirculation. In this case, the process according to the invention advantageously comprises a step in which the said unthickened sludge from the settling tank of the type producing unthickened sludge is mixed with sludge recirculated from the lamellar thickener settling tank with sludge recirculation.
According to one variant of the invention, the process consists of passing water to be treated through the said settling tank producing unthickened sludge and then in the said thickener settling tank, the said settling tanks then being supplied in series.
According to another variant, the said settling tank producing unthickened sludge and the said thickener settling tank can be configured in parallel.
This type of variants to the process will be used to adapt it to the conditions in which it is used and particularly to variations in the material and water flows to be treated.
According to another interesting aspect of the invention, the process comprises an additional step consisting of adding at least one reagent in order to improve settling and thickening of the sludge in the said thickener settling tank. This reagent may be added continuously to the water, in other words the said reagent is added directly into the influent to be treated or in a coagulation-flocculation step included on the inlet side of the said thickener settling tank, or included in it. Advantageously, it may be envisaged as part of an installation using the ECOFLOC prolonged coagulation process developed by the applicant and described in French patent FR 2 631 021.
The main advantages of this invention that consists of the combined use of a settling tank of the type producing unthickened sludge (preferably a settling tank with flocs ballasted by at least one granular material) and a thickener settling tank (preferably lamellar with or without sludge recirculation) are as follows.
The process according to the invention is more competitive than processes according to standard practice mentioned in the preamble (settling tank + thickener or settling tank with integrated thickener) particularly due to the fact that it reduces the dimensions of installations necessary for its embodiment.
This invention eliminates the need to return overflows from thickeners to the beginning of the installation, which are necessary when settling tanks coupled with a sludge thickener are used alone.
Furthermore, if unthickened but pre-flocculated sludge obtained from the ballasted floc settling tank is injected into the thickener settling tank, it is possible to:
obtain a higher concentration of SS (suspended solids) at the inlet to the thickener settling tank, that enables optimized hydraulic treatment velocities within this thickener settling tank;
obtain sludge with a higher dryness ratio;
if applicable, reduce the recirculated sludge flow rate when it is decided to use a thickener settling tank with sludge recirculation;
possibly reduce the content of reagents used in the thickener settling tank;
increase the load on the thickener settling tank base mat (since the sludge is mixed, it thickens more quickly so that the load on the base mat of the thickener part of the thickener settling tank can be increased and therefore improved) while keeping the water quality conform with the required quality;
easily accommodate variations in pollutant flows to be treated;
adapt the process to treat dry weather or wet weather water and material flows.
The invention also relates to any installation for use of the process as described above, characterized in that it comprises means of conveying the water to be treated to the inlet, at least one settling tank of the type producing unthickened sludge, at least one thickener settling tank, means of transporting at least part of the unthickened sludge from the settling tank of the type producing unthickened sludge to the thickener settling tank, means of conveying treated water away from the said settling tank of the type producing unthickened sludge and from the said thickener settling tank and means of conveying thickened sludge away from the said thickener settling tank.
If required, this type of installation could be preceded by conventional means for preliminary treatment of the water to be treated (for example screening, sand removal and degreasing, or simply screening). Sludge from thickened sludge removal means can be conveyed in a known manner to means of treatment of this sludge. Finally, a post-treatment can then be applied to treated water outlet from the installation.
Preferably, said means of conveyance of water to be treated of the installation according to the invention are such that the said settling tank of the type producing unthickened sludge and the said thickener settling tank can be configured either in series or in parallel. This type of installation could thus be easily configured to adapt to variations in content and/or flow of the influent. Advantage could also be taken of this characteristic when the installation is started up.
Preferably, the said settling tank of the type producing unthickened sludge is a lamellar settling tank with flocs ballasted by at least one granular material (for example such as an ACTIFLO settling tank marketed by the Applicant).
Also preferably, the said thickener settling tank is a lamellar thickener settling tank or a lamellar thickener settling tank with sludge recirculation.
Advantageously, the installation also comprises means of adding at least one coagulating or flocculating reagent in the said thickener settling tank.
The invention also relates to processes for use of this installation.
In particular, the invention also relates to a process that consists of supplying the said settling tank of the type producing unthickened sludge and the said thickener settling tank in series when starting up of the installation, and supplying the said settling tanks in parallel once the installation has been started up.
The invention also relates to a process that consists of supplying the said settling tank of the type producing unthickened sludge and the said thickener settling tank in series when a normal flow and/or inlet of water to be treated is input into the said installation, and supplying the said settling tanks in parallel during peak water or material flows.